1. Field of the Invention
This invention is in the field of making thin ferroelectric films comprising PT, PZT, or PLZT with temperature of crystallization of 450.degree., 500.degree. or 500.degree. C., respectively. The invention also relates to acetate-based aqueous solutions, methods for making ferroelectric films and devices utilizing such films.
2. Description of the Prior Art
There has been considerable interest in the production of ferroelectric lead perovskites such as PbTiO.sub.3 (PT), Pb.sub.x (Zr.sub.y, Ti.sub.1-y)O.sub.3 (PZT) and (Pb.sub.1-x, La.sub.x)(Zr.sub.y,Ti.sub.1-y).sub.1-x/4 O.sub.3 (PLZT) owing to their unusual dielectric, pyroelectric, piezoelectric and nonlinear optical properties: (C. E. Land, P. D. Thatcher, and G. H. Haertling, "Electrooptic Ceramics," in Appl. Solid State Sci., Vol. 4, R. Wolfe, Ed., Academic, New York, 1974, P. 137-233). Applications include high value capacitors (Y. Shintani and O. Tada, "Preparation of Thin Barium Titanate Films" by D. C. Diode Sputtering, J. Appl. Phys., 41, 2376-80 (1970)), optical switches (K. Wasa, O. Yamazaki, H. Adachi, T. Kawaguchi, and K. Setsune, "Optical TIR Switches Using PLZT Thin-Film Waveguides on Sapphire," IEEE J. Light Wave Tech. LT., 2,710-14 (1984)), displays and memories (Y. Hamakawa, Y. Matsui, Y. Higuma, and T. Nakagawa, "A Nonvolatile Memory FET Using PLT Thin Film Gate" in Technical Digest IEEE International Electron Devices Meeting (Electron Devices Society, The Institute of Electrical and Electronics Engineers, Washington, D.C., 1977), p. 294-7).
Recent research on PT/PZT/PLZT perovskites has extended from bulk materials to thin films (H. Adachi, T. Kawaguchi, K. Stesune, K. Ohji, and K. Wasa, "Electro-Optic Effects of (Pb,La)(Zr,Ti)O.sub.3 Thin Films Prepared by rf Planar Magnetron sputtering," Appl. Phys. Lett., 42, 867-68 (1983)), (K. D. Budd, S K. Dey and D. A. Payne, "Sol-gel Processing of Lead Titanate (PbTiO.sub.3), Lead Zirconate (PbZrO.sub.3), PZT, and PLZT Thin Films," Proc. British Ceram. Soc., 36, 107-21 (1985)), (R. W. Vest and J. Xu, "Preparation and Properties of PLZT Films from Metallo-Organic Precursors," Ferroelectrics, 93, 21-29 (1989), (G. H. Haertling, "An Acetate Process for bulk and Thin Film PLZT," the Seventh International Symposium on the Applications of Ferroelectrics, University of Illinois at UrbanaChampaign, Jun. 6-8, 1990). These thin films have been successfully synthesized from metal alkoxides via sol-gel (K. D. Budd, S K. Dey and D. A. Payne, "Sol-gel Processing of Lead Titanate (PbTiO.sub.3), Lead Zirconate (PbZrO.sub.3), PZT, and PLZT Thin Films," Proc. British Ceram. Soc., 36, 107-21 (1985)), fatty-acid salts of metals via metallo-organic decomposition (MOD), (R. W. Vest and J. Xu, "Preparation and Properties of PLZT Films from Metallo-Organic Precursors," Ferroelectrics, 93,21-29 (1989), and metal acetate precursors via aqueous solution techniques, (G. H. Haertling, "An Acetate Process for bulk and Thin Film PLZT," the Seventh International Symposium on the Applications of Ferroelectrics, University of Illinois at Urbana-Champaign, Jun. 6-8, 1990). These processes have the advantage over other techniques of greater purity and compositional control with better homogeneity of mixing and lower processing temperatures. In the metal acetate precursors via aqueous solution technique, titanium acetylacetonate or titanium oxide-acetyl acetonate have been precursors for synthesizing PLZT thin films which crystallize at 700.degree. C. This temperature is far from the ideal one (.about.500.degree. C.) suitable for the device fabrication integrated with semiconductor processing. Other unsolved problems of these chemical processes are (1) the long-term stability of the formulated precursor solutions, (2) the substrate wetting (i.e., a poor film-substrate adhesion), and (3) film cracking (often due to a low metals/organics ratio leading to extensive film shrinkage on pyrolysis of the organics). U.S. Pat. No. 4,990,324 (Okira) describes a method for making 2 or 3 component lead zirconate-titanate systems. U.S. Pat. No. 4,963,390 describes method for depositing lead titanate (PT), lead zirconate titanate (PZT) and PZT doped films such as with lanthanum (PLZT). U.S. Pat. No. 4,832,893 describes a methods for making PLZT. U.S. Pat. No. 4,810,484 describes method for manufacturing fine lead titanate powders.